I have solved this problem by using a short length of heat-schrink tubing which is available from most electronic/electrical parts suppliers and model shops in various lenths, diameters and colours. Choose a diameter slightly larger than the external diamenter of the threaded nipple and about 2-3 mm longer than the nipple and before shrinking it put a small amount of instant glue on the thread. After shrinking trim the end of the tubing so that it covers the end of the nipple. It is important to make sure that the nipple is very clean before sliding on the heat-shrink tubing and I used a brass wire brush for this purpose.